1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus.
2. Description of the Related Art
Hitherto, as an image forming apparatus such as a copying machine, a laser beam printer, a facsimile machine, and a multifunction peripheral having functions of those apparatus, there is an image forming apparatus including a sheet processing apparatus configured to perform processes such as a binding process and a sorting process on sheets having images formed thereon. Further, there is widely known an image forming apparatus (image forming system) including the sheet processing apparatus connected to a discharge port formed in a main body of the image forming apparatus so as to automatically perform the above-mentioned processes on the sheets on-line.
As the sheet processing apparatus, there is widely used a sheet processing apparatus configured to perform the processes such as a binding process on a sheet bundle, which is formed by stacking a plurality of sheets onto an intermediate processing tray provided inside the sheet processing apparatus. For example, there is an apparatus configured to bind the sheet bundle by a binding unit at the time of the binding process, discharge the bound sheet bundle from the intermediate processing tray by a bundle discharge roller pair serving as a rotary member pair, and stack the discharged sheet bundle onto a stacking tray (see Japanese Patent Application Laid-Open No. 2012-96869).
When a first sheet is to be stacked onto the intermediate processing tray, an upper bundle discharge roller of the bundle discharge roller pair is lowered so as to nip the sheet between the upper bundle discharge roller and a lower bundle discharge roller. Then, the upper bundle discharge roller is rotated in a reverse direction so as to convey the sheet toward a trailing edge stopper provided on the intermediate processing tray. When a second or subsequent sheet is to be stacked, the upper bundle discharge roller is moved upwardly so as not to interfere with the succeeding sheet sliding toward the trailing edge stopper along an upper surface of the sheet, which is already stacked on the intermediate processing tray. When the sheet bundle is bound and the bound sheet bundle is to be discharged, the upper bundle discharge roller is lowered so as to nip the sheet bundle between the upper bundle discharge roller and the lower bundle discharge roller, to thereby discharge the sheet bundle.
When the first sheet is to be stacked onto the intermediate processing tray in the related-art sheet processing apparatus, the bundle discharge roller pair is rotated in the direction opposite to the rotational direction at the time of discharging the sheet onto the stacking tray. In this case, the drive configuration of the bundle discharge roller pair is set so that the drive is first transmitted to the lower bundle discharge roller and then transmitted to the upper bundle discharge roller via a drive transmission unit including a plurality of gears. That is, when the sheet bundle is to be discharged onto the stacking tray in the related-art sheet processing apparatus, the lower bundle discharge roller is first rotated, and the rotation of the lower bundle discharge roller is then transmitted via the drive transmission unit so that the upper bundle discharge roller is rotated.
In the case of this drive configuration, when the bound sheet bundle is to be discharged onto the stacking tray, during the rotation of the lower bundle discharge roller in the sheet discharge direction, backlashes of the drive transmission unit may cause a delay in the drive transmission to the upper bundle discharge roller. As a result, the start of rotation of the upper bundle discharge roller is delayed as compared to the lower bundle discharge roller.
Thus, when the sheet bundle is to be discharged, a lower sheet of the sheet bundle, which is held in contact with the lower bundle discharge roller, is conveyed but, due to the delay in the start of rotation of the upper bundle discharge roller, an upper sheet of the sheet bundle, which is held in contact with the upper bundle discharge roller, is not conveyed at this time, with the result that the upper sheet is distorted. When the drive is then transmitted to the upper bundle discharge roller, the sheet bundle is conveyed under a state in which the upper sheet is distorted. When the sheet bundle is conveyed in this state, the binding portion of the sheet bundle passes through the bundle discharge roller pair without a relief region for the distorted portion of the upper sheet. The distorted portion is crushed by the bundle discharge roller pair, with the result that creases may be generated in the binding portion.
To prevent the generation of creases, there is conceived a method of reducing a press-contact force of the bundle discharge roller pair to suppress the distortion of the upper sheet when the bundle discharge roller pair abuts against the sheet bundle and conveys the sheet bundle. When the conveyance force of the bundle discharge roller pair is excessively reduced, however, in a case of a thick sheet bundle, the conveyance force is insufficient, which may cause slippage between the bundle discharge rollers and the sheet bundle during the conveyance of the sheet bundle by the bundle discharge roller pair. When the sheet bundle is slipped, marks of slippage are generated on the surface of the sheet bundle by the bundle discharge roller pair, with the result that the quality of a product may be degraded.